In the end, a joint House-Senate conference committee will likely hammer out the final version of the bill. That might not take place until a "lame duck" session after the November election, when much of the political pressure on lawmakers has dissipated.

Which means that despite the oft-repeated assertion by Sen. Lindsay Graham (R-S.C.) that "cap and trade is dead," the House's bill based on cap and trade could be back in play -- someday, given the right conditions. Even if they do not enact cap and trade, Democratic leaders could use a conference to ratchet up the climate regulations past what the Senate agreed to and beyond what Democratic House centrists want.

"We have a lot of wiggle room in conference," said a House Democratic aide.

And it could be hard for centrists in either party or either chamber to walk away from the bill if they have taken the risk of voting for it on initial passage.

"Once you get to conference, it's an up-or-down vote," said Norm Ornstein, a veteran congressional expert at the American Enterprise Institute. "People who vote against it have to explain why they voted for it before they voted against it."

Still, no lawmakers have publicly suggested a strategy in which Senate Democrats pass a watered-down bill with hopes to beef it up in conference. But Democratic leaders say they do expect the final version of the bill to be negotiated in a conference committee. That has been a rare occurrence since Democrats took over Congress in 2007. Parliamentary maneuvering and partisan clashes have often meant that one chamber is forced to accept the version of the other after leadership works out complicated deals, such as in the case of health care.

The idea of leaving the heavy lifting for the conference committee is an unwelcome one in the Senate Democratic caucus, from centrist Sen. Joe Lieberman (I-Conn.) to Sen. Bernie Sanders (I-Vt.), possibly the most liberal member of the Senate.

"Members of the Senate have their views as to what constitutes a strong bill, and they're going to want to be heard on this," Sanders said.

But House Majority Leader Steny Hoyer (D-Md.) acknowledged that lawmakers on the conference committee may wind up merging the House cap-and-trade plan with a Senate bill that does not include it.

"I have always said we need to have a conference on this issue," Hoyer said last week. "If there are differences, that is fine, but we need to resolve those. We need to resolve them in the context of the Senate taking action, which we have always indicated, whether it is on just an energy independence portion or energy and climate."

The mix of options Reid will have to find 60 Senate votes is enough to frustrate a Las Vegas card counter. There are a range of outcomes commonly discussed for the energy debate, but there is no clear path to any of them -- even doing nothing.

It is not simply a question of what policies Reid will try to pass, but how. Will he attach climate provisions to the legislative response to the oil spill, widely considered a "must pass" by lawmakers who want to show voters they have taken action on the BP spill? Would he add the oil spill provisions to the "energy only" legislation? Or will he seek a "comprehensive" bill that includes some form of emissions regulation pricing carbon, along with measures to satisfy the traditional and renewable energy industries.

One theory, popular among Republicans, is that the only thing Democrats will be able to pass is an oil spill response, restructuring the now-renamed Minerals Management Service inside the Interior Department, tightening regulations and changing the limits on what oil companies can be required to pay for a spill. Such a bill is widely considered to be "must pass" legislation.

Others predict oil spill legislation will be added to the energy-only bill, which gives the environmental community the renewable energy standard, requiring ordering utilities to use a set amount of renewable electricity by a certain date.

A majority of Senate Democrats last week rallied for a "comprehensive" bill. And Democratic senators say Reid plans to combine oil spill legislation with energy legislation, hoping to make the case that a vote against the bill is a vote for BP and "Big Oil" (E&E Daily, June 25).

But that presents another problem. Lawmakers will probably want to return home for the November election with proof for voters that they have done something to try to prevent future spills. But most members, particularly vulnerable Democrats in conservative districts, would rather vote on any controversial energy bill after the election, when election-year political pressures are gone.

Driving some sense of urgency is that Congress is almost out of time. Though the year is not yet half over, Congress has little more than about eight weeks of legislative activity before the election. That is one reason why many expect the final energy or climate bill to be worked out during the lame-duck session between the November election and the start of the new Congress in January.

"If we get our act together," Sanders said, "we have the time -- although time is short -- we do have the time to do it."